The present invention relates to a drive circuit for an ultrasonic motor, in particular a drive circuit for an ultrasonic motor for producing a high voltage alternating current necessary for operating a progressive or traveling type ultrasonic motor and for suppressing a noise when the ultrasonic motor is stopped.
Heretofore, various ultrasonic motors have been developed, wherein a traveling type ultrasonic motor among them is operated according to the following theory. In detail, when a vertical wave and a lateral wave are synthesized on a surface of an elastic member as a stator to form a traveling wave transmitted in a longitudinal direction of the elastic member at a sonic speed, the surface of the elastic member starts elliptically vibrating due to the traveling wave. When a rotor contacts the surface of the elliptically vibrating elastic member under pressure, the rotor contacts peaks of the elastic member by the elliptical vibrations, and starts moving in a direction opposite to the traveling wave at a vibrating speed by friction between the rotor and the stator.
In order to form the traveling wave on the elastic member as the stator of the traveling type ultrasonic motor as described above, a curved elastic wave is used. A method for generating the curved elastic wave is described hereunder. As shown in FIG. 14(a), the stator a is formed by bonding a piezoelectric member b, i.e. piezoelectric ceramic, where the adjacent components are alternately polarized in the thickness direction, to the elastic member c. In the drawing, the respective arrows in the piezoelectric member b show the polarized directions.
In case a high frequency voltage is applied to the piezoelectric ceramic b, if the applied frequency is equal to an inherent frequency of the stator a, the stator a resonates to make curved vibrations in a circumferential direction as shown in FIG. 15. In case the vibrating waves of the stator a are divided into, for example, nine wavelengths in the circumferential direction as shown in FIG. 15, nine wave peaks m.sub.1 to m.sub.9 are formed and called standing waves.
On the other hand, as shown in FIGS. 14(b) and 16, in case phases of mutual positions of section A and section B of the piezoelectric ceramic 2 are shifted by 3/4 wavelength (.lambda.) of the applied high frequency voltage, and high frequency voltages .PHI..sub.0, .PHI..sub.90 having a timewise phase difference of 90.degree. are applied to the section A and section B, respectively, the standing waves are generated by the respective high frequency voltages .PHI..sub.0, .PHI..sub.90 as described above. These standing waves interfere with each other to be synthesized, so that the traveling wave is formed.
Incidentally, while an alternating current of high frequency voltages .PHI..sub.0, .PHI..sub.90 having a timewise phase difference of 90.degree. as described above is required as an electric power necessary for operating the ultrasonic motor, in order to raise an efficiency of the ultrasonic motor, it is also necessary to allow the alternate current to have an optimum frequency. Also, it is required to improve reliability of the drive circuit for the ultrasonic motor. Moreover, it is required for the drive circuit for the ultrasonic motor to easily produce the alternating current.
Further, the alternating current with high frequency voltages .PHI..sub.0, .PHI..sub.90 is preferably controlled by pulse control. When the ultrasonic motor is controlled by the pulse control, in order to stop the ultrasonic motor, the pulse may be stopped. When the pulse is stopped, as shown in FIG. 17, the electric power supplied to the ultrasonic motor is shut off instantaneously.
On the other hand, during the operation of the ultrasonic motor, the rotor R rotates by the traveling wave based on the elliptical vibration of the stator S. At this time, as shown in FIG. 18, the rotor R is lifted by the heads of the traveling wave in the stator S from other portions of the stator S. When the motor apply voltage is suddenly shut off by stopping the pulse, as shown in FIG. 19, the stator S stops the elliptical vibration immediately to extinguish the traveling wave. Thus, a situation appears such that the stator S and rotor R are instantaneously spaced from each other. However, since the rotor R is pressed against the stator S by a spring H, after the condition as shown in FIG. 19, the rotor R closely contacts the stator S, as shown in FIG. 20. At this time, there is produced a noise when the rotor R contacts the stator S, which is referred to hereinafter as "stopping noise" in the specification.
In this case, since the vibration sound of the ultrasonic motor is relatively low, the stopping noise is relatively noticeable. Sometimes, the stopping noise causes uncomfortable feeling.
In consideration of the above-mentioned requirements and situation, the present invention has been made, and an object of the invention is to provide a drive circuit for an ultrasonic motor, wherein an alternating current for driving the ultrasonic motor can be easily produced from a direct current.
Another object of the invention is to provide a drive circuit for an ultrasonic motor as stated above, wherein an alternating current to be applied to the ultrasonic motor has an optimum frequency.
A further object of the invention is to provide a drive circuit for an ultrasonic motor as stated above, wherein a reliability of the drive circuit for the ultrasonic motor can be improved.
A still further object of the invention is to provide a drive circuit for an ultrasonic motor as stated above, wherein the stopping noise of the ultrasonic motor is reduced as little as possible to thereby provide quietness in all the operation range of the ultrasonic motor.
Further objects and advantages of the invention will be apparent from the following description of the invention.